Histogram equalization, in which a histogram of the cumulative numbers of pixels having the same gradation value in a one-screen input image is constructed and the gradation scale is transformed to equalize the distribution, has been proposed (in Patent Document 1, for example) as a method of improving the gradation curve of an image.
The so-called retinex method, in which a weighted average of spatial luminance variations is calculated from an input image, the weighted average is converted to a logarithm, and an improved luminance signal is calculated from this logarithm and the logarithmic values of the input image, thereby improving the dynamic range of the image, has also been proposed (in Non-Patent Document 1 and Patent Document 2, for example).
Patent Document 1: Japanese Patent Application Publication No. 2002-27285 (paragraphs 0029-0041, FIG. 1)
Patent Document 1: Japanese Patent Application Publication No. 2005-38119 (paragraphs 0028-0031, FIG. 1)
Non-Patent Document: Z. Rahman et al., “A Multiscale Retinex for Color Rendition and Dynamic Range Compression”, XIX Proc. SPIE, Vol. 2847, pp. 183-191, Nov. 1996